thats because they are not steamer inserts, and the items sold in the uk as billy cans were never designed to be cooking vessels.
the billycans sold in the UK are imported from Thailand where they are used for there intended purpose, they are the Asian equivalent of a lunch box.
Asians travelling or going to work store there lunch in these containers, the part you refer to as a 'steamer insert' is in fact a divider so that the smaller portion of the food (meat and sauce for example) can be kept separate from the rice in the larger compartment.
you can drill holes in this divider and convert it into a steamer though
I have seen other items sold as billy cans which are in fact ice buckets for drinks bottles etc these however do not have a divider